ACEPHALA. 243 



in the sand, are furnished with a tongue-like foot, 

 by which they dig the holes wherein they lie con- 

 cealed, and crawl, or even leap about, upon the 

 shore. Many, as the Pholades, penetrate the solid 

 rocks and stones, and excavate therein the caverns 

 that they inhabit ; or, in the case of the Teredo, 

 with dangerous industry bore into the bottoms of 

 ships, or submerged wood of any description, and 

 silently destroy by their insidious ravages, the piers 

 or dykes which human labour has erected."* 



This Class of Mollusca is destitute of a distinct 

 head, the mouth being opened in the body within 

 the folds of the double mantle, which enclose the 

 animal, as the covers of a book do the leaves. 

 Enclosing the two folds of the mantle are found, 

 almost universally, two valves of a shell, united at 

 one part of their edge by an elastic hinge. Cuvier, 

 it is true, has placed in this Class certain genera 

 which are merely enveloped in a sort of cartila- 

 ginous or leathery bag, open at each end ; these 

 agree somewhat in their general structure with the 

 inhabitants of Bivalves; but there is sufficient dis- 

 crepancy to have induced other naturalists to consi- 

 der them as forming a very distinct group. The 

 whole Class is essentially aquatic in habits. 



The best method of understanding the structure 

 of these animals will be to take a common species, 

 and carefully observe what we see there. There 

 is none more suitable for such an examination 

 than the Scallop. 



* Jones, Anim. Kingd, p. 375. 



M 2 



